This invention relates to thyristor pulse control circuits.
More particularly, the invention relates to thyristor pulse control circuits of the kind comprising a main thyristor for connection between a load and a d.c. source, means for gating the thyristor into conduction, a commutation circuit including a commutating capacitor and means for charging the commutating capacitor and for connecting the charged capacitor across the main thyristor so as to reverse bias and thus commutate it, and a bypass contactor connected in parallel with the main thyristor so that on closure of the bypass contactor the load is connected direct to the d.c. source.
The bypass contactor enables the range of current which can be handled by the control circuit to be extended beyond that which would be economically feasible using the main thyristor alone. One of the problems associated with the operation of a bypass contactor is the arcing which occurs at the tips of the contactor when the tips open to break a high current; such arcing reduces the life of the contact or tips and so increases the maintenance required for the contactor. Arcing can also occur on closure of the contactor if the main thyristor is in a non-conducting state when the contactor tips make contact. One way of solving this problem, for example as described in United Kingdom Pat. No. 1,018,645, is to ensure that the main thyristor is gated immediately before contactor tips close and also immediately before the tips open, so that the main thyristor carries the load current at the instant the tips close or open and no arcing can occur. This is practicable since, although it is expensive to construct a thyristor chopper controller capable of continuous operation at very high currents, it is possible at relatively little expense to ensure that a controller can occasionally commutate very high currents. However, if the controller is to operate with a relatively low voltage source, for example if it is to control the d.c. traction motor of an industrial truck equipped with a 24 volt battery, the charge on the commutating capacitor may be insufficient to commutate the high current carried by the main thyristor after opening of the bypass contactor under high current conditions, unless the size of the capacitor is increased far beyond that required to commutate normal currents, thus increasing the cost of the controller. This problem also occurs if a long period of time elapses between closure and opening of the bypass contactor, during which time the charge of the commutating capacitor can be reduced by leakage to a low value.